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Male-female differences in peafowl

The biology of gender is scientific analysis of
the physical basis for behavioural differences between men and
women. It is more specific than sexual dimorphism, which covers
physical and behavioural differences between males and females of
any sexually reproducing species, or sexual differentiation, where
physical and behavioural differences between men and women are
described.

Research in this area is generally motivated by the search for
causes of diseases in human
beings, and ways of treating or preventing those diseases; it is
thought that men and women might require different kinds of
treatment for certain diseases. The results are relevant to gender
issues, but that is not their direct concern.

History

The late twentieth century saw an explosion in technology
capable of aiding sex research. Scientists made great progress
towards understanding the formation of gender identity in humans. Extensive advances were also made in
understanding sexual dimorphism in other animals. For example,
there were studies on the effects of sex hormones on rats. The early twenty
first century started producing even more amazing results
concerning genetically programmed sexual
dimorphism in rat brains, prior even to the influence of hormones
on development. "Genes on the sex chromosomes can directly influence
sexual dimorphism in cognition and behaviour, independent of the action of sex
steroids."[4]

Differences

Brain

Human brain

The brains of many animals, including humans, are significantly different for males and females of the species.[5] Both genes and hormones affect the formation
of many animal brains before "birth" (or hatching), and also behaviour of adult
individuals. Hormones significantly affect human brain formation,
and also brain development at puberty. Both kinds of brain
difference affect male and female behaviour.

It is commonly accepted that there are many sex-related
differences in behavior in the human species. These differences and
their neurobiological bases have been sought. Gur et al.
suggested that women have a higher percentage of grey matter (GM) in
comparison to men, and that men have a higher percentage of white matter (WM) and
of cerebrospinal fluid (CSF). The
white matter of 20 year men consists of around 176,000 km of myelinatedaxons compared to 149,000 km in women of
the same age.[7]
In Gur's study, in men the percentage of GM was higher in the left
hemisphere, the percentage of WM was symmetric, and the percentage
of CSF was higher in the right hemisphere. In women, no asymmetry
was shown. Gur et al. stated that the higher percentage of
GM in women could compensate for less space in the cranium,
since GM is used for computation more than transfer of information
across distant regions. They state that the sex-based differences
in percentage and asymmetry of GM and WM in the brain may account
for the differences in cognitive functioning of the two sexes. They
also stated:[8]

“

The anatomic results
suggest some parallels between sex differences in cognition and
differences in GM because both women and the left language
hemisphere have higher percentage of GM, and women outperform men
on language tasks. [...] Considered separately, the performance
data replicated earlier reports of better verbal relative to
spatial performance in women as compared with men, against overall
similar levels of average performance.[8]

”

Haier and colleagues used voxel-based morphometry (VBM) to
identify brain areas where clusters GM and WM volumes are located
in correlation to the FSIQ test. In 2004, Haier et al. claimed
that:

“

These findings support
the view that individual differences in grey and white matter
volumes, in a relatively small number of areas distributed
throughout the brain, account for considerable variance in
individual differences in general intelligence.[9]

”

However, both neuronal cell bodies (grey matter) and axons
(white matter) are essential to the function of the nervous system,
so the functional implication of having more of one or the other is
not clear.

Aptitude

A 2001 report by Richard J. Coley of the ETS stated, "A review of
the elementary and secondary education achievement data included in
this report from NAEP found that
females in all racial/ethnic groups scored higher, on average, than
males in reading, writing, and civics. There was an advantage found
in science for Hispanic and White males. In mathematics,
essentially no differences between males and females were
found."[10]

Kiefer and Sekaquaptewa proposed that a source of some women's
underperformance and lowered perseverance in mathematical fields is
these women's underlying "implicit" sex-based stereotypes regarding
mathematical ability and association, as well as their
identification with their gender.[11]

A number of studies have looked for sex differences in the brain
that might relate to sex differences in intelligence or performance
on different tasks. These studies have included measures of total
brain size, relative amounts of grey and white matter, and a wide
variety of measures of brain activity patterns (Sex and Intelligence). However, findings of
sex differences in the brain do not answer the Nature versus Nurture controversy raised
again by Summers' comments, because studies of neuroplasticity
show that the brain can be altered by experience.

In mathematical reasoning, Benbow et al. stated of a 1983
study:

“

When graphed (Benbow,
1988), the male and female SAT-V
[verbal] distributions were found to be essentially equivalent, but
the male SAT-M [math] distributions manifested a higher mean and
larger variance than was observed for the females. Consequently, an
exponential intensification of the male:female ratio occurred in
the upper tail of the combined distribution. The ratio was 2:1 for
adolescents with SAT-M scores of at least 500, 4:1 for those with
scores of at least 600, and 13:1 for those with scores of at least
700.[12]

”

In 1983, Benbow stated of the study, "The results obtained by
both procedures establish that by age 13 a large sex difference in
mathematical reasoning ability exists and that it is especially
pronounced at the high end of the distribution [...]."[13]

Baron-Cohen states that the male:female ratio of autism is 4:1, and examines
autism beginning from a theory of the "male brain type," a
hypothetical construct defined as "an individual whose folk physics
skills are in advance of his or her social folk psychology skills.
That is, they show a folk physics>folk psychology discrepancy.
This is regardless of one’s chromosomal sex."[14]
Baron-Cohen's theory and findings are controversial and many
studies contradict the idea that baby boys and girls differ
significantly in the way they learn or reason about objects'
mechanical interactions.[15]

Aggression

Campbell argues that "female competition is more likely to take
the form of indirect aggression or low-level direct combat than
among males." Campbell goes on to state that "cultural
interpretations have 'enhanced' evolutionarily based sex
differences by a process of imposition which stigmatises the
expression of aggression by females and causes women to offer
exculpatory (rather than justificatory) accounts of their own
aggression."[16]
Research has shown that stimulation of the amygdala induces a delayed and prolonged
increase of aggressiveness in male Syrian golden hamsters,[17] and
in the hypothalamus of male rats.[18] Many
studies have examined the correlation between aggression and
certain hormones and neurotransmitters, specifically testosterone.
However, the link between testosterone and aggression in humans
remains unclear.[19][20][21] A
more established negative correlation has been discovered between
serotonin and aggression, meaning that higher levels of serotonin
are correlated with lower levels of aggression and vice versa.[22]

See also

Brain Sex, a book by
Anne Moir (geneticist) and David Jessel (journalist)